The present invention relates generally to launch vehicles and to spacecraft and, more particularly, to a reusable single-stage-to-orbit spacecraft and a reusable launch assist platform.
There is a considerable and increasing need for some type of transportation system to launch satellites into space for various purposes, such as communications, weather forecasting, earth sensing, and microgravity research. Currently, space transportation uses multistage rockets that successively discard their lower stages. Only the last stage reaches space and, if unmanned, becomes space debris or disintegrates when reentering the atmosphere.
One purpose of the space shuttle system was to avoid wasting all these stages with a reusable spacecraft and partially reusable rockets. Unfortunately, the shuttle has hardly met its goal of reducing waste. First, the shuttle uses solid propellant booster rockets and a large expensive fuel tank for a takeoff. The large tank is never reused, and the booster rockets are only reused after extensive refurbishing. NASA reuses the last stage of the shuttle, called the orbiter, but orbiters can only be reused after extensive refurbishment. The loss of hardware and the cost of refurbishment is one reason the shuttle has not achieved its promise to make space accessible.
The cost of space transportation by any means is enormous. A common cost figure is six to ten thousand dollars to lift every pound to orbit. This means that one conventional rocket launch may cost over one hundred million dollars, and one space shuttle trip costs more than five hundred million dollars. These exorbitant costs are due in part to the use and ultimate loss of many stages including the last stage.
To overcome the disadvantages and expense of current space systems, many scientists propose reusable, single-stage-to-orbit (SSTO) rocket vehicles. Despite some studies on SSTO designs, and papers describing different design approaches, most experts doubt whether the concept is feasible with available technologies. Although efforts in this area have spanned more than a decade, no SSTO has yet reached orbit. The reason for this is the extreme mass fraction required to achieve orbit.
The mass fraction, which is the ratio of fuel to total weight required at lift-off, is determined by the basic rocket equation, m=m.sub.o e.sup.v/c, where m is the mass reaching orbit, m.sub.o is the mass at lift-off, v is the theoretical velocity gain required for orbit, and c is the exhaust velocity of the rocket engine.
The theoretical velocity to reach orbit is 9 to 10 km per second, depending on the size of the vehicle and its aerodynamic losses. Given conventional launch techniques, this forces the end mass reaching orbit to be only about 10% of the lift-off mass.
Designing and building a space vehicle to carry not only useful payload, but also fuel weighing ten times the weight of the payload and space vehicle, including some safety margin, is difficult enough. Adding the requirement that the vehicle return to earth and not disintegrate from the reentry heat makes the design task nearly impossible with today's technology. Most space science experts believe that this task requires further advances in materials and rocket engines.
Government research efforts have funded the DC-X Program and several preliminary launches of a test vehicle in the New Mexico desert. The DC-X Program has witnessed takeoff and landing of a reusable vehicle, as reported in Aviation Week & Space Technology, pp. 46-49, (Oct. 11, 1993). The DC-X, however, only flew about 100 feet above ground before returning to the ground under the power of its liquid oxygen/hydrogen rocket engines.
Another proposed SSTO craft under development, the Delta Clipper, has a slender conical design. The Delta Clipper is designed to reenter the earth's atmosphere nose-first, and ultimately land in a vertical, nose-up position. There are, however, no conventional engines with the thrust to launch and return the Delta Clipper.
Both of these designs confront the same problems. Any space vehicle expends an inordinate amount of fuel (between 30 and 50% depending on vehicle size and trajectory) in the early part of the trajectory just to fight earth's gravity and air resistance. These two factors combine to make space transportation very difficult.
Providing for reentry adds yet another problem. Sleek, economical vehicle designs may reduce drag on launch, but they are ill-suited for reentry. On the other hand, an SSTO using a large, blunt base for atmospheric reentry (similar to the Mercury, Gemini, and Apollo capsules) causes severe air drag losses during ascent through the atmosphere.
To solve these problems, some scientists have proposed different types of launch systems. Many systems use airplanes specially designed to bring space vehicles aloft. This approach has several drawbacks. Because airplanes cannot fly above the atmosphere, any SSTO launched from airplanes would still need to traverse a substantial amount of atmosphere before reaching space.
Furthermore, an airplane must travel at some minimum horizontal airspeed to obtain lift from its wings. This speed, however, provides little help to launch an SSTO to space.
In addition, if the SSTO were designed to have a broad base for reentry, it would have a typical weight of 100,000 to 500,000 lbs. and would not fit inside conventional airplanes. This limitation would require a new plane with mammoth dimensions.
The literature also mentions some platforms propelled by jet engines. Some of these, however, only lift objects over hurdles.
One which does not is U.S. Pat. No. 3,285,175. The device in that patent has several jet engines arranged in a circle to lift a missile into the atmosphere before launching. This system is limited, though, by its use of jet engines. Jet engines run out of sufficient air to carry their own weight at altitudes of 5,000 to 6,000 meters, far below the minimum altitude required (20,000 meters) to launch a SSTO without a large atmospheric drag.
Another disadvantage of these systems is that some are not fully reusable because they cannot be recovered easily. Launch systems often return to earth unpowered, making it difficult to recover them.
Conventional spacecraft are also poorly designed for reuse. Few, if any, unmanned vehicles return to earth under their own power. Moreover, as explained above, the design of spacecraft capable of reentering the atmosphere greatly complicates the design of launch vehicles.